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<div class="titlepage"><div><div><h3 class="title">
<a name="context.rationale.other_apis_"></a><a class="link" href="other_apis_.html" title="Other APIs">Other APIs </a>
</h3></div></div></div>
<h5>
<a name="context.rationale.other_apis_.h0"></a>
        <span class="phrase"><a name="context.rationale.other_apis_.setjmp___longjmp__"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.setjmp___longjmp__">setjmp()/longjmp()</a>
      </h5>
<p>
        C99 defines <code class="computeroutput"><span class="identifier">setjmp</span><span class="special">()</span></code>/<code class="computeroutput"><span class="identifier">longjmp</span><span class="special">()</span></code>
        to provide non-local jumps but it does not require that <span class="emphasis"><em>longjmp()</em></span>
        preserves the current stack frame. Therefore, jumping into a function which
        was exited via a call to <span class="emphasis"><em>longjmp()</em></span> is undefined <a href="#ftn.context.rationale.other_apis_.f0" class="footnote" name="context.rationale.other_apis_.f0"><sup class="footnote">[2]</sup></a>.
      </p>
<a name="ucontext"></a><h5>
<a name="context.rationale.other_apis_.h1"></a>
        <span class="phrase"><a name="context.rationale.other_apis_.ucontext_t"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.ucontext_t">ucontext_t</a>
      </h5>
<p>
        Since POSIX.1-2004 <code class="computeroutput"><span class="identifier">ucontext_t</span></code>
        is deprecated and was removed in POSIX.1-2008! The function signature of
        <code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code>
        is:
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">makecontext</span><span class="special">(</span><span class="identifier">ucontext_t</span> <span class="special">*</span><span class="identifier">ucp</span><span class="special">,</span> <span class="keyword">void</span> <span class="special">(*</span><span class="identifier">func</span><span class="special">)(),</span> <span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="special">...);</span>
</pre>
<p>
        The third argument of <code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code> specifies the number of integer arguments
        that follow which will require function pointer cast if <code class="computeroutput"><span class="identifier">func</span></code>
        will accept those arguments which is undefined in C99 <a href="#ftn.context.rationale.other_apis_.f1" class="footnote" name="context.rationale.other_apis_.f1"><sup class="footnote">[3]</sup></a>.
      </p>
<p>
        The arguments in the var-arg list are required to be integers, passing pointers
        in var-arg list is not guaranteed to work, especially it will fail for architectures
        where pointers are larger than integers.
      </p>
<p>
        <code class="computeroutput"><span class="identifier">ucontext_t</span></code> preserves signal
        mask between context switches which involves system calls consuming a lot
        of CPU cycles (ucontext_t is slower; a context switch takes <a class="link" href="../performance.html#performance"><span class="emphasis"><em>two
        magnitutes of order more CPU cycles</em></span></a> more than <span class="emphasis"><em>fcontext_t</em></span>).
      </p>
<h5>
<a name="context.rationale.other_apis_.h2"></a>
        <span class="phrase"><a name="context.rationale.other_apis_.windows_fibers"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.windows_fibers">Windows
        fibers</a>
      </h5>
<p>
        A drawback of Windows Fiber API is that <code class="computeroutput"><span class="identifier">CreateFiber</span><span class="special">()</span></code> does not accept a pointer to user allocated
        stack space preventing the reuse of stacks for other context instances. Because
        the Windows Fiber API requires to call <code class="computeroutput"><span class="identifier">ConvertThreadToFiber</span><span class="special">()</span></code> if <code class="computeroutput"><span class="identifier">SwitchFiber</span><span class="special">()</span></code> is called for a thread which has not been
        converted to a fiber. For the same reason <code class="computeroutput"><span class="identifier">ConvertFiberToThread</span><span class="special">()</span></code> must be called after return from <code class="computeroutput"><span class="identifier">SwitchFiber</span><span class="special">()</span></code>
        if the thread was forced to be converted to a fiber before (which is inefficient).
      </p>
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span> <span class="identifier">is_a_fiber</span><span class="special">()</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">ConvertThreadToFiber</span><span class="special">(</span> <span class="number">0</span><span class="special">);</span>
    <span class="identifier">SwitchToFiber</span><span class="special">(</span> <span class="identifier">ctx</span><span class="special">);</span>
    <span class="identifier">ConvertFiberToThread</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
        If the condition <code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">&gt;=</span> <span class="identifier">_WIN32_WINNT_VISTA</span></code>
        is met function <code class="computeroutput"><span class="identifier">IsThreadAFiber</span><span class="special">()</span></code> is provided in order to detect if the current
        thread was already converted. Unfortunately Windows XP + SP 2/3 defines
        <code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">&gt;=</span>
        <span class="identifier">_WIN32_WINNT_VISTA</span></code> without providing
        <code class="computeroutput"><span class="identifier">IsThreadAFiber</span><span class="special">()</span></code>.
      </p>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.context.rationale.other_apis_.f0" class="footnote"><p><a href="#context.rationale.other_apis_.f0" class="para"><sup class="para">[2] </sup></a>
          ISO/IEC 9899:1999, 2005, 7.13.2.1:2
        </p></div>
<div id="ftn.context.rationale.other_apis_.f1" class="footnote"><p><a href="#context.rationale.other_apis_.f1" class="para"><sup class="para">[3] </sup></a>
          ISO/IEC 9899:1999, 2005, J.2
        </p></div>
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<td align="right"><div class="copyright-footer">Copyright © 2014 Oliver Kowalke<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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